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<h2 class="title">
<a id="synonyms-expand-or-contract"></a>扩展或收缩<a class="edit_me edit_me_private" rel="nofollow" title="Editing on GitHub is available to Elastic" href="https://github.com/elasticsearch-cn/elasticsearch-definitive-guide/edit/cn/260_Synonyms/40_Expand_contract.asciidoc">edit</a>
</h2>
</div></div></div>
<p>在 <a class="xref" href="synonym-formats.html" title="同义词格式">同义词格式</a> 中，我们看到了可以通过 <em>简单扩展</em> 、 <em>简单收缩</em> 、或_类型扩展_ 来指明同义词规则。
本章节我们将在这三者间做个权衡比较。</p>
<div class="tip admon">
<div class="icon"></div>
<div class="admon_content">
<p>本节仅处理单词同义词。多词同义词又增添了一层复杂性，在 <a class="xref" href="multi-word-synonyms.html" title="多词同义词和短语查询">多词同义词和短语查询</a> 中，我们将会讨论。</p>
</div>
</div>
<div class="section">
<div class="titlepage"><div><div>
<h3 class="title">
<a id="synonyms-expansion"></a>简单扩展<a class="edit_me edit_me_private" rel="nofollow" title="Editing on GitHub is available to Elastic" href="https://github.com/elasticsearch-cn/elasticsearch-definitive-guide/edit/cn/260_Synonyms/40_Expand_contract.asciidoc">edit</a>
</h3>
</div></div></div>
<p>通过 <em>简单扩展</em> ，我们可以把同义词列表中的任意一个词扩展成同义词列表 <em>所有</em> 的词：</p>
<pre class="literallayout">"jump,hop,leap"</pre>

<p>扩展可以应用在索引阶段或查询阶段。两者都有优点
(⬆)︎ 和缺点 (⬇)︎。到底要在哪个阶段使用，则取决于性能与灵活性：</p>
<div class="informaltable">
<table border="1" cellpadding="4px">
<colgroup>
<col class="col_1">
<col class="col_2">
<col class="col_3">
</colgroup>
<thead>
<tr>
<th align="left" valign="top"></th>
<th align="left" valign="top">索引</th>
<th align="left" valign="top">查询</th>
</tr>
</thead>
<tbody>
<tr>
<td align="left" valign="top"><p><span class="strong strong"><strong>索引的大小</strong></span></p></td>
<td align="left" valign="top"><p>⬇︎ 大索引。因为所有的同义词都会被索引，所以索引的大小相对会变大一些。</p></td>
<td align="left" valign="top"><p>⬆︎ 正常大小。</p></td>
</tr>
<tr>
<td align="left" valign="top"><p><span class="strong strong"><strong>关联</strong></span></p></td>
<td align="left" valign="top"><p>⬇︎ 所有同义词都有相同的 IDF（至于什么是 IDF ，参见 <a class="xref" href="relevance-intro.html" title="什么是相关性?">什么是相关性?</a>），这意味着通用的词和较常用的词都拥有着相同的权重。</p></td>
<td align="left" valign="top"><p>⬆︎ 每个同义词 IDF 都和原来一样。</p></td>
</tr>
<tr>
<td align="left" valign="top"><p><span class="strong strong"><strong>性能</strong></span></p></td>
<td align="left" valign="top"><p>⬆︎ 查询只需要找到查询字符串中指定单个词项。</p></td>
<td align="left" valign="top"><p>⬇︎ 对一个词项的查询重写来查找所有的同义词，从而降低性能。</p></td>
</tr>
<tr>
<td align="left" valign="top"><p><span class="strong strong"><strong>灵活性</strong></span></p></td>
<td align="left" valign="top"><p>⬇︎ 同义词规则不能改变现有的文件。对于有影响的新规则，现有的文件都要重建（注：重新索引一次文档）。</p></td>
<td align="left" valign="top"><p>⬆︎ 同义词规则可以更新不需要索引文件。</p></td>
</tr>
</tbody>
</table>
</div>
</div>

<div class="section">
<div class="titlepage"><div><div>
<h3 class="title">
<a id="synonyms-contraction"></a>简单收缩<a class="edit_me edit_me_private" rel="nofollow" title="Editing on GitHub is available to Elastic" href="https://github.com/elasticsearch-cn/elasticsearch-definitive-guide/edit/cn/260_Synonyms/40_Expand_contract.asciidoc">edit</a>
</h3>
</div></div></div>
<p><em>简单收缩</em> ，把  左边的多个同义词映射到了右边的单个词：</p>
<pre class="literallayout">"leap,hop =&gt; jump"</pre>

<p>它必须同时应用于索引和查询阶段，以确保查询词项映射到索引中存在的同一个值。</p>
<p>相对于简单扩展方法，这种方法也有一些优点和一些缺点：</p>
<div class="variablelist">
<dl class="variablelist">
<dt>
<span class="term">
索引的大小
</span>
</dt>
<dd>
⬆︎ 索引大小是正常的，因为只有单一词项被索引。
</dd>
<dt>
<span class="term">
关联
</span>
</dt>
<dd>
⬇︎ 所有词项的 IDF 是一样的，所以你不能区分比较常用的词、不常用的单词。
</dd>
<dt>
<span class="term">
性能
</span>
</dt>
<dd>
⬆︎ 查询只需要在索引中找到单词的出现。
</dd>
<dt>
<span class="term">
灵活性
</span>
</dt>
<dd>
<p>⬆︎ 新同义词可以添加到规则的左侧并在查询阶段使用。例如，我们想添加 <code class="literal">bound</code> 到先前指定的同义词规则中。那么下面的规则将作用于包含 <code class="literal">bound</code> 的查询或包含 <code class="literal">bound</code> 的文档索引：</p>
<pre class="literallayout">"leap,hop,bound =&gt; jump"</pre>

<p>似乎对旧有的文档不起作用是么？其实我们可以把上面这个同义词规则改写下，以便对旧有文档同样起作用：</p>
<pre class="literallayout">"leap,hop,bound =&gt; jump,bound"</pre>

<p>当你重建索引文件，你可以恢复到上面的规则（注： <code class="literal">leap,hop,bound =&gt; jump</code> ）来获得查询单个词项的性能优势（注：因为上面那个规则相比这个而言，查询阶段就只要查询一个词了）。</p>
</dd>
</dl>
</div>
</div>

<div class="section">
<div class="titlepage"><div><div>
<h3 class="title">
<a id="synonyms-genres"></a>类型扩展<a class="edit_me edit_me_private" rel="nofollow" title="Editing on GitHub is available to Elastic" href="https://github.com/elasticsearch-cn/elasticsearch-definitive-guide/edit/cn/260_Synonyms/40_Expand_contract.asciidoc">edit</a>
</h3>
</div></div></div>
<p>类型扩展是完全不同于简单收缩  或扩张，
并不是平等看待所有的同义词，而是扩大了词的意义，使被拓展的词更为通用。以这些规则为例：</p>
<pre class="literallayout">"cat    =&gt; cat,pet",
"kitten =&gt; kitten,cat,pet",
"dog    =&gt; dog,pet"
"puppy  =&gt; puppy,dog,pet"</pre>

<p>通过在索引阶段使用类型扩展：</p>
<div class="ulist itemizedlist">
<ul class="itemizedlist">
<li class="listitem">
一个关于 <code class="literal">kitten</code> 的查询会发现关于 kittens 的文档。
</li>
<li class="listitem">
查询一个 <code class="literal">cat</code> 会找到关于 kittens 和 cats 的文档。
</li>
<li class="listitem">
一个 <code class="literal">pet</code> 的查询将发现有关的 kittens、cats、puppies、dogs 或者 pets 的文档。
</li>
</ul>
</div>
<p>或者在查询阶段使用类型扩展， <code class="literal">kitten</code> 的查询结果就会被拓展成涉及到 kittens、cats、dogs。</p>
<p>您也可以有两全其美的办法，通过在索引阶段应用类型扩展同义词规则，以确保类型在索引中存在。然后，在查询阶段，
你可以选择不采用同义词（使 <code class="literal">kitten</code> 查询只返回 kittens 的文件）或采用同义词， <code class="literal">kitten</code> 的查询操作就会返回包括 kittens、cats、pets（也包括 dogs 和 puppies）的相关结果。</p>
<p>前面的示例规则，对 <code class="literal">kitten</code> 的 IDF 将是正确的，而 <code class="literal">cat</code> 和 <code class="literal">pet</code> 的 IDF 将会被 Elasticsearch 降权。然而, 这是对你有利的，当一个针对 <code class="literal">kitten</code> 的查询被拓展成了针对 <code class="literal">kitten OR cat OR pet</code> 的查询， 那么 <code class="literal">kitten</code> 相关的文档就应该排在最上方，其次是 <code class="literal">cat</code> 的文件， <code class="literal">pet</code> 的文件将被排在最底部。</p>
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